20th Century Fox offers a novel alternative to midnight mass on Christmas Eve: midnight showings of “Aliens vs. Predator: Requieum” an 86-minute horror sequel starring nobody you ever heard of but featuring a cross between the titular creatures. Moviegoers at the Union Square in Manhattan and the Chinese Theater in LA will receive T-shirts emblazoned with “I survived Midnight-Mass-Acre Christmas Eve 2007.” Classy! Does Bill Donohue know about this? 20th naturally isn’t showing “Requieum” to real critics in advance, but they did let fanboy reviewer Gil Brooks have a look and he regifted them with a 3.5 star (out of 5) review. The movie’s first five minutes are also online if you just can’t wait until midnight. “Requiem” should drain box-office blood from “Sweeney Todd,” which dropped 28 percent from Friday to Saturday, slashing a projected $12 million weekend take to $9.4 million. Anne Thompson at Variety blames Paramount’s marketing, which makes “Sweeney” resemble a straight horror movie, much to the annoyance of fans who were taken aback to discover themselves sitting through a Stephen Sondheim musical.